Zero-Configuration Assignment of IPv6 Multicast Addresses Using mDNS

Internet-Draft	Zeroconf Assignment of IPv6 Mcast Addrs	September 2026
Karstens, et al.	Expires 26 March 2027	[Page]

Abstract

This document describes a zero-configuration protocol for dynamically assigning IPv6 multicast addresses that are unique at the link-layer. Applications randomly assign multicast group IDs from a specified range and prevent collisions by using Multicast DNS (mDNS) to publish resource records under a new "eth-addr.arpa" domain. This protocol satisfies all of the criteria listed in RFC 10019.¶

1. Introduction

[RFC10019] includes a problem statement and requirements for a zero-configuration method for dynamically assigning multicast addresses. This document describes a process that fulfills these requirements by having applications randomly assign IPv6 multicast group IDs from a specified range and using mDNS [RFC6762] to prevent collisions in both IPv6 and link-layer addresses.¶

Note that DNS-based Service Discovery (DNS-SD) [RFC6763] uses several different DNS resource record types, published using either Unicast or Multicast DNS, to facilitate service discovery. This document uses a single DNS resource record type (PTR), published using Multicast DNS (mDNS), to coordinate IPv6 multicast address assignment in a zero-configuration environment. The DNS resource records in this protocol may be published alongside records for other domain name services, such as DNS-SD, or they may be published alone. mDNS is used rather than a new protocol with the expectation that functionality for address assignment can be achieved using existing mDNS implementations.¶

The protocol described in this document is well-suited for networks that rely on IPv6 multicast and already deploy mDNS. This document makes IANA assignments for use on Ethernet and other protocols based on the IEEE 802 architecture. It may be adapted for use with other link layers, but that is outside the scope of this document.¶

As mentioned in [RFC6762], Section 21, mDNS is an algorithm that assumes cooperating participants. Due to its reliance on mDNS, this assumption applies to the protocol described in this document as well.¶

Scalability of mDNS is an area of ongoing research and improvement (see [RFC7558]). The protocol described in this document is subject to similar constraints. Under normal operating conditions, mDNS traffic is expected to grow linearly with the number of multicast streams.¶

The protocol uses implicit availability: if an application does not receive any indication that an address is in use, then it assumes that the address is available for its own use. Because this coordination occurs using mDNS, any filtering of mDNS traffic by the host or network would prevent applications from coordinating use of multicast addresses. This may result in address collisions.¶

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

2. Procedure

This section describes how an application dynamically generates an IPv6 multicast address, uses mDNS to coordinate use of that address with other applications on the network, and begins transmitting its multicast stream.¶

Because this protocol is focused specifically on allocating IPv6 multicast addresses, applications MUST send and receive mDNS messages using the IPv6 multicast address for mDNS. In order to be compatible with existing mDNS implementations that require IPv4, applications MAY also send and receive mDNS messages using the IPv4 multicast address for mDNS.¶

mDNS implementations used with this protocol MUST coordinate with all applications using the same mDNS implementation, as well as with other mDNS implementations running on the same host. Such coordination ensures that conflicts arising from independently generated, identical group IDs are detected during the probing phase, allowing the affected applications to select alternate group IDs. Note that coordination between mDNS implementations running on the same host tends to happen naturally if transmitted multicast packets are looped back into the receive pipeline.¶

The process an application uses from initialization to transmitting a multicast stream is divided into a series of steps, outlined below:¶

2.1. Group ID Selection

When an application is preparing to transmit a multicast stream, it begins by selecting an IPv6 multicast group ID.¶

If the application is either 1) transmitting a multicast stream for the first time or 2) responding to an address collision, then the application SHALL select a random multicast group ID value from the range 0x90000000-0x9FFFFFFF. IANA is REQUESTED to assign this range from the "Dynamic Multicast Group IDs" registry (see IANA Considerations).¶

If the application has previously transmitted the multicast stream, then the selected multicast group ID should have been retained in persistent storage (see Optimization). The application SHALL proceed using the stored value.¶

Note that the range of possible group ID values means that the probability that another application has chosen the same group ID is (n / 2²⁸), where n is the number of applications on the network.¶

2.2. Address Generation

Once the IPv6 multicast group ID has been selected, the application SHALL use this to generate two multicast addresses:¶

IPv6 Multicast Address¶

This is a link-scoped IPv6 multicast address, calculated by combining the multicast group ID with the Interface Identifier (IID) of the intended source address for the multicast stream, according to the format given in [RFC4489], Section 3.¶
Multicast Ethernet Address¶

This is the corresponding multicast Ethernet address that will be used to transmit the data. It is calculated from the IPv6 multicast address, as described in [RFC2464], Section 7.¶

For example, consider a source address of fe80::a12:34ff:fe56:7890. If the selected group ID is 9abc:def0, then the calculated IPv6 multicast address would be ff32:ff:a12:34ff:fe56:7890:9abc:def0 and the multicast Ethernet address would be 33:33:9A:BC:DE:F0.¶

2.3. Domain Name Assembly

Once the multicast Ethernet address is determined, the application SHALL use this to assemble a domain name by encoding nibbles from the multicast Ethernet address as a sequence of lower-case hexadecimal digits, separated by dots, in reverse order, and ending with the suffix ".eth-addr.arpa" (a new domain in the .arpa registry).¶

Considering the example in Address Generation, the resulting domain name would be "0.f.e.d.c.b.a.9.3.3.3.3.eth-addr.arpa".¶

Note that this is similar to how DNS resource records used for reverse mapping IPv6 addresses are created (see [RFC3596], Section 2.5). As such, this document reserves the domain "9.3.3.3.3.eth-addr.arpa" as a special-use domain in the "Special-Use Domain Names" registry (see IANA Considerations). The "3.3.3.3" component of this is taken from the 33:33 prefix used for mapping IPv6 multicast addresses to Ethernet multicast addresses and the "9" component indicates a multicast group ID in the range 0x90000000-0x9FFFFFFF.¶

2.4. Record Construction

Once the domain name is assembled, the application SHALL construct a unique PTR record using that domain name. The PTRDNAME field of this record SHALL consist of a unique application identifier, in the form of one or more DNS labels, followed by the device's host name (for example, "application.example.local."). Integrating a unique identifier in this manner allows for multiple applications to be on the same host.¶

Note that A/AAAA records may also be published for this same host name (for example, "example.local."), though this is not a requirement for this design.¶

To allow for the creation of veto records (see Veto Records), first DNS label of the application identifier SHALL NOT exceed 58 octets in length. This reflects the 63-byte limit specified in [RFC1035], Section 2.3.4 while also reserving space for a 5-byte suffix.¶

2.5. Probing

After the PTR record is constructed, the application SHALL use the mDNS probing algorithm described in [RFC6762], Section 8.1 to probe for a PTR record with the same name.¶

Due to the large range of values for group IDs, it is unlikely that two applications would randomly choose the same group ID and begin probing at the same time. However, in that event, the conflict SHALL be resolved according to the procedure in [RFC6762], Section 8.2. The loser of the conflict SHALL return to Group ID Selection and choose a different group ID.¶

2.6. Announcing and Responding

If the probing algorithm started in Probing completes without any conflict, then the application SHALL announce the PTR record according to the procedure in [RFC6762], Section 8.3 and SHALL respond to queries for the PTR record according to [RFC6762], Section 6.¶

2.7. Conflict Detection and Resolution

The application SHALL start a continuous query for the PTR record according to [RFC6762], Section 5.2. This helps detect a conflict in the event of a network partition and repair (see Collision Detection After Partition Repair). If a conflict is detected at any time, then the application SHALL resolve the conflict according to [RFC6762], Section 9. The losing application SHALL stop transmitting that multicast stream and return to Group ID Selection, choosing a new group ID. As before, this new group ID is also retained in persistent storage, overwriting any group ID previously saved for this multicast stream.¶

The host network stack may optionally monitor the network for traffic that uses the same destination multicast Ethernet address, but a different destination multicast IPv6 address. If this is detected, then the application SHALL stop transmitting that multicast stream and return to Group ID Selection, choosing a new group ID. Only one application need respond in this manner to resolve the collision, but both may do so. No method to coordinate this between hosts is necessary and this protocol does not specify one.¶

2.8. Transmitting

Once the PTR record is advertised and the continuous query started, the host MAY then begin transmitting multicast data using the multicast addresses from Address Generation.¶

2.9. Optimization

The application SHALL retain the group ID value in persistent storage and use it the next time the multicast stream is transmitted. This allows the network to quickly settle on a configuration and reduces the likelihood of future collisions, as long as the network is unchanged.¶

Note that retaining the value does not permit the application to skip any of the preceding steps. This is consistent with [RFC6762], Section 10.2 and allows the application to recover from any network changes that may have occurred when it was offline.¶

2.10. Defending

The application SHALL defend the PTR record from probes as described in [RFC6762], Section 6.¶

3. Veto Records

Veto records are used to address collisions that occur in the network infrastructure (for example, the address table in a switch is discussed in [RFC10019], Section 2).¶

When a network infrastructure component detects a collision it cannot resolve, it triggers a conflict with the application by publishing a veto record. A veto record is a unique PTR record using the string generated for the address as its name. The veto record's PTRDNAME field is created by appending the string "-veto" to the first DNS label that appears in the PTRDNAME field of the PTR record that caused the address collision (for example, "application-veto"). This technique results in a veto PTR record that is always lexicographically later than the PTR record from the application (specifically, the first DNS label is always longer, so the first byte of the RDATA, which contains the length, always contains a larger value). In turn, this causes the veto record to always win the conflict resolution described in [RFC6762], Section 9.¶

Network infrastructure components SHALL publish veto records without probing.¶

Applications SHALL respond to the conflict the same as to a collision, eventually resulting in a new group ID being acquired. As before, the application retains its new group ID in persistent storage, ensuring the same conflict is not repeated in the future. As such, it is unnecessary to store vetoes to persistent storage.¶

If an application transmitting a veto record determines that the PTR record for the address causing the conflict has become invalid (either due to receiving a goodbye packet or to record expiry), then it SHALL query the network for the PTR record. If the application does not receive a query response containing the PTR record after 5 seconds, then the application SHALL delay a random amount of time in the range 20-120 ms and then send a goodbye packet that invalidates the veto record.¶

5. Evaluation of Solution

[RFC10019] contains a list of criteria to evaluate potential solutions. The following is an analysis of how this protocol satisfies the requirements listed in that document:¶

[REQ-1]: Unique Address Assignment: Use of the protocol results in a unique address being assigned to the multicast group at both the network and link layers.¶
[REQ-2]: Resilience to Single Points of Failure: The protocol uses mDNS, which uses a peer-to-peer communication model and so has no single points of failure, as long as network connectivity is not interrupted.¶
[REQ-3]: Zero User Configuration: The protocol operates without requiring user or administrator configuration.¶
[REQ-4]: Coexistence with Multicast Address Allocation Solutions: This document assigns a range of group IDs from the "Dynamic Multicast Group IDs" registry for use with the protocol, which allows the protocol to coexist with other multicast IP address allocation solutions (see [RFC10028]).¶
[REQ-5]: Single-Subnet Operation: The protocol supports operation within a single IPv6 subnet.¶
[REQ-6]: No External Connectivity: The protocol does not require Internet access or connectivity to external infrastructure.¶
[REQ-7]: Supports Multiple Host Applications: Multiple applications on the same host are supported by 1) incorporating a unique application identifier into the PTR record's PTRDNAME field and by 2) requiring the mDNS implementation(s) on a host to facilitate interaction between those applications.¶
[REQ-8]: Collision Detection and Resolution: The protocol includes a mechanism to detect and resolve multicast address collisions at both the network and link layers. See Collision Detection After Partition Repair for additional discussion about detecting and resolving collisions after a network partition is repaired.¶

The following is an analysis of how this protocol satisfies the desirable characteristics listed in [RFC10019]:¶

[CONS-1]: Multi-Subnet Support: The protocol can operate across multiple subnets (see Use on Networks with Multiple Subnets).¶
[CONS-2]: Standards Compatibility: The protocol uses existing protocols without requiring any changes.¶
[CONS-3]: Cross-Platform Availability: The protocol uses mDNS, which is implemented on many host platforms and operating systems.¶
[CONS-4]: Minimal Dependency on Manufacturing Data: The protocol does not rely on pre-loaded configuration or device-specific manufacturing data.¶
[CONS-5]: Low Overhead: The protocol uses mDNS, which is designed to minimize the volume and frequency of network traffic generated during normal operation.¶
[CONS-6]: Advertisement: This document does not prescribe any mechanism for advertising the IPv6 address used for the multicast stream. However, because the protocol already uses mDNS, a natural option for doing so would be to use DNS-SD ([RFC6763]) to advertise a "_udp" service and publish the address used for the multicast stream in a TXT record.¶
[CONS-7]: Network Topology: Veto records ensure that the protocol works regardless of the underlying topology and adjacencies.¶

5.1. Collision Detection After Partition Repair

A network partition occurs when one part of a network is temporarily unable to communicate with another part of the network. This may happen, for example, if a switch goes offline. A partition repair happens when communication is fully restored to all parts of the network.¶

Because mDNS is designed to be a low-bandwidth protocol, it can take a signficant amount of time to detect a resource record collision after a network partition is repaired. This is not a concern on networks where all multicast streams are established before any likely partition event because all group IDs will have been selected and stored for future use.¶

It is a greater concern on networks where multicast streams may be established at any time. Deployments on these networks may consider engaging supplemental detection and resolution mechanisms. Specifics of this functionality are open to future enhancement, but are considered out-of-scope for this document.¶

6. IANA Considerations

IANA should allocate a block of group IDs from the "Dynamic Multicast Group IDs" registry in the "IPv6 Multicast Address Space" registry group that was created by [RFC10028]. The range of this block should be 0x90000000-0x9FFFFFFF and the description should be "Zero-Configuration Assignment of IPv6 Multicast Addresses Using mDNS".¶

IANA manages two registries that contain entries used for mapping addresses to names. This document introduces "eth-addr.arpa" for use in a similar capacity and so requests that it be added to these registries as well.¶

The domain "eth-addr.arpa" should be registered in the .arpa registry (https://www.iana.org/domains/arpa). The usage should be "For mapping Ethernet addresses to local domain names for Zero-Configuration Assignment of IPv6 Multicast Addresses" and the reference should be [I-D.draft-ietf-pim-ipv6-zeroconf-assignment].¶

The special-use domain "9.3.3.3.3.eth-addr.arpa." should be registered in the "Special-Use Domain Names" registry (https://www.iana.org/assignments/special-use-domain-names). This domain should not be delegated.¶

6.1. Domain Name Reservation Considerations

[RFC6761], Section 5 includes a list of questions that must be answered when reserving a new Special-Use Domain Name. The answers to these questions for the "9.3.3.3.3.eth-addr.arpa" Special-Use Domain, and any names falling within this domain, are as follows:¶

Users are free to use these names as they would for any other reverse mapping domain. Because this mapping is closely-related to the local network, users SHOULD be aware that these names are likely to yield different results on different networks.¶
Application software SHOULD recognize these names as a reverse mapping domain and MAY associate them with the protocol described in this document.¶
Name resolution APIs and libraries SHOULD recognize these names as being used exclusively with mDNS, and so SHOULD NOT send queries for these names to their configured unicast DNS server(s).¶
Caching DNS servers SHOULD recognize these names as being used exclusively with mDNS, SHOULD NOT attempt to look up resource records associated with these names, and SHOULD respond to any query with NXDOMAIN.¶
Authoritative DNS servers SHOULD recognize these names as being used exclusively with mDNS and SHOULD respond to any query with NXDOMAIN.¶
DNS server operators MUST NOT configure an authoritative DNS server to answer queries for these names.¶
DNS Registries/Registrars MUST NOT register "9.3.3.3.3.eth-addr.arpa" names.¶

9. References

9.1. Normative References

[RFC1035]: Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2464]: Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998, <https://www.rfc-editor.org/info/rfc2464>.
[RFC3306]: Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306, September 2002, <https://www.rfc-editor.org/info/rfc3306>.
[RFC4489]: Park, J., Shin, M., and H. Kim, "A Method for Generating Link-Scoped IPv6 Multicast Addresses", RFC 4489, DOI 10.17487/RFC4489, April 2006, <https://www.rfc-editor.org/info/rfc4489>.
[RFC6761]: Cheshire, S. and M. Krochmal, "Special-Use Domain Names", RFC 6761, DOI 10.17487/RFC6761, February 2013, <https://www.rfc-editor.org/info/rfc6761>.
[RFC6762]: Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, DOI 10.17487/RFC6762, February 2013, <https://www.rfc-editor.org/info/rfc6762>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC10028]: Karstens, N., Farinacci, D., and M. McBride, "Updates to Dynamic IPv6 Multicast Address Group IDs", RFC 10028, DOI 10.17487/RFC10028, August 2026, <https://www.rfc-editor.org/info/rfc10028>.

9.2. Informative References

[RFC3596]: Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", STD 88, RFC 3596, DOI 10.17487/RFC3596, October 2003, <https://www.rfc-editor.org/info/rfc3596>.
[RFC6763]: Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, <https://www.rfc-editor.org/info/rfc6763>.
[RFC7558]: Lynn, K., Cheshire, S., Blanchet, M., and D. Migault, "Requirements for Scalable DNS-Based Service Discovery (DNS-SD) / Multicast DNS (mDNS) Extensions", RFC 7558, DOI 10.17487/RFC7558, July 2015, <https://www.rfc-editor.org/info/rfc7558>.
[RFC7942]: Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, <https://www.rfc-editor.org/info/rfc7942>.
[RFC8815]: Abrahamsson, M., Chown, T., Giuliano, L., and T. Eckert, "Deprecating Any-Source Multicast (ASM) for Interdomain Multicast", BCP 229, RFC 8815, DOI 10.17487/RFC8815, August 2020, <https://www.rfc-editor.org/info/rfc8815>.
[RFC10019]: Karstens, N., Farinacci, D., and M. McBride, "Zeroconf Multicast Address Allocation Problem Statement and Requirements", RFC 10019, DOI 10.17487/RFC10019, July 2026, <https://www.rfc-editor.org/info/rfc10019>.